This invention relates to methods for electrically connecting a flip chip to a substrate.
Flip chip mounting is an increasingly popular technique for directly electrically connecting an integrated circuit chip to a substrate such as a circuit board. In this configuration, the active face of the chip is mounted face down, or "flipped" on the substrate. The electrical bond pads on the flip chip are aligned with corresponding electrical bond pads on the substrate, with the chip and substrate bond pads electrically connected by way of an electrically conductive material. The flip chip mounting technique eliminates the use of bond wires between a chip or chip package and the substrate, resulting in increased reliability of the chip-to-substrate bond.
A wide range of electrically conducting compositions have been proposed for making the interconnection between flip chip and substrate bond pads. Solder balls, gold bumps, gold stud bumps, and other conventional metal bump configurations have been used extensively. Aside from metallic compositions, electrically conducting polymer compositions are gaining wide acceptance as flip chip interconnection bump materials. In a flip chip mounting technique employing polymer interconnections, electrically conductive polymer bumps are formed on the bond pads, typically of the flip chip, and are polymerized or dried during bonding to the substrate bond pads, whereby both an electrical and a mechanical adhesive bond between the flip chip and the substrate bond pads is produced. Electrically conductive polymer materials are particularly well-suited for flip chip mounting techniques because of their ease of application, because they eliminate many of the unwanted characteristics of metallic interconnections, e.g., solder flux, and because for some polymer materials reworkability of faulty flip chips is enabled by simple heating of the material.
Conventionally, once a flip chip is bonded to a substrate, whether by metallic or by polymer bump interconnections between the chip and substrate bond pads, an underfill material is dispensed between the chip and the substrate. The underfill material is typically provided as a liquid adhesive resin that can be dried or polymerized. The underfill material provides enhanced mechanical adhesion and mechanical and thermal stability between the flip chip and the substrate, and inhibits environmental attack of chip and substrate surfaces.